Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of from about 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and non-exposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the non-exposed regions remain on the substrate.
Various radiation compositions and imageable elements containing reactive polymer binders are described in U.S. Pat. No. 6,569,603 (Furukawa) and EP 1,182,033A1 (Fujimaki et al.). Other IR-sensitive compositions are described in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,787,281 (Tao et al.), and U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Patent Application Publication 2003/0118939 (West et al.), and EP 1,079,276A1 (Lifka et al.) and EP 1,449,650A1 (Goto).
Wrapping and conditioning (heat-treatment) of multi-layer positive-working imageable elements has been carried out to reduce variability in sensitivity over time as described for example in WO 99/21715 (McCullough et al.) and U.S. Pat. No. 6,706,466 (Lott et al.).
Problem to be Solved
The various radiation sensitive compositions of the art can readily be used to prepare negative-working imageable elements. However, there can be an alteration or variability in on-press developability and printing quality when the radiation-sensitive composition is carried on an aluminum substrate that has been anodized using sulfuric acid, and the element is subsequently exposed to a humid environment. In particular, these elements may exhibit “background staining” from undesirably retained imaging coating in non-exposed regions when high moisture is present in the environment in which they are stored or used.
There is a need to address this problem and to provide negative-working imageable elements with sulfuric acid-anodized substrates that exhibit little or no background staining under the noted conditions.